Constraint solving for function overloading

[auht]

No description provided.

[LPTK]

Note: normally, even for draft PRs, we try to maintain a state where all the tests are green. For test cases that currently don't work, we still use the corresponding :e (= expect a type error here) but together with a TODO/FIXME comment.

[LPTK]

Should probably raise a proper error here, which should refer to the failing TSC.

[LPTK]

TODO:

• update getVarsPol
• update Constrained class with ew field to use for TSCs
• update App typer case to do the TSC introduction there (and not in constraint solver)

[LPTK]

@auht Please make sure to address every comment that was reported in this PR and then promptly close it (since #213 subsumes it). It's been needlessly lingering for a long time.